Method and a System for Energy Recovery and/or Cooling

ABSTRACT

The present invention relates to a method and a system of energy recovery and/or cooling in at least one electrolysis cells ( 1 ) for the production of a metal, in particular aluminium, where the cell(s) is provided with one or more heat exchangers and where a heat exchange medium circulates through said heat exchanger(s) and is further directed to at least one heat conversion unit, such as an open circuit expander turbine ( 3 ). The expander turbine is connected with a compressor ( 2 ) that supplies the heat exchange medium to the cells ( 1, 51 ). In one embodiment, a second expander turbine ( 40 ) is connected to a generator ( 41 ) that can be applied for production of electricity. The system is substantially self propelled and the heat exchange medium is preferably air applied in an open circuit.

The present invention relates to a method and a system for energy recovery and/or cooling of electrolysis cells for metal production, where excess heat is extracted from the cells to cool the cells and/or to be used in an advantageous manner.

In particular the invention comprises a method and a system for recovery of surplus heat from electrolysis cells in the aluminium industry, and the conversion of the recovered heat energy into other forms of energy such as pressure and possibly electrical energy. Further, the heat exchange medium at an elevated pressure can be utilised to produce electrical energy and this energy can be fed to the cells to increase the production volume of said cells and/or to reduce the electrical power consumption from the ordinary grid. Pressurised medium can be introduced into the cell for cooling purposes.

In production of aluminium the energy consumption due to the electrolysis process will be approximately 13 kWh electrical energy plus consumption of about 0.4 kg (theoretical 0.334 kg C=3.15 kWh) carbon pr. kg aluminium produced. The theoretical enthalpy required per kg aluminium produced is approximately 6.4 kWh. Thus, there is a loss of energy in the cells due to the fact that the current that passes through the electrolyte generates excessive heat mainly due to the ohmic resistance of the electrolyte.

In electrolysis cells for the production of aluminium, it is necessary to keep a frozen ledge of solid electrolyte along the sidewalls of the cell cavity to prevent degrading and erosion of the cell lining material. If it should happen that this ledge melts down as a result of elevated temperature in the cell, the cells life-span will be highly affected.

On the other hand, if the cell becomes too cold in particular along its bottom part, the cathode will be successively covered by frozen electrolyte or sludge that reduces the active area for current distribution and consequently the electric resistance will raise and energy efficiency will decrease because of unfavourable magnetohydrodynamic effects.

Typically the thermal balance and thus the thickness of the ledge of an individual cell is regulated by modifying the cell resistance through the distance between anode and cathode, while the amperage is the same for all cells in a serially connected row of cells. Introducing heat extraction systems in electrolysis cells involves a challenge with regard to keeping the balance between energy supplied to the cell and the cooling of each cell. On the other hand it allows a more flexible operation of cells including higher amperages and better reaction to variations in the amperage or parameters affecting the cooling conditions, like ambient temperature.

When an electrolysis cell for aluminium production is optimally operated, there is a mainly proportional relation between the amperage of the electric energy supplied and the amount of aluminium produced. Increasing the amperage requires to keep or even increase the resistance of the electrolyte to maintain magnetohydrodynamic stability, if the cell was operated at lowest possible voltage. Thus the energy supplied to the cell is increasing close to quadratic with production if no other modifications to the cell are applied. As a simplifying consideration an increase in the production can be allowed as long as the temperature in the cell is controlled by the removal of excess heat from the cell. The excess heat can for instance be collected by allowing a cooling medium to circulate in a closed circuit that is heat exchanged with electrolyte and melted metal in heat exchangers. Such heat exchangers can be implemented in the sidewalls of the cell and in the bottom thereof.

In the prior art, there have been proposed several solutions to recover or use the excessive heat that is produced in aluminium electrolysis cells.

Commonly, a relatively small part of the heat is at present recovered and for instance used to preheat feed-materials and anodes to keep up with the process temperature that normally is within the interval 950-965° C.

WO 87/00211 discloses a cell arrangement for electrometallurgical purposes, where cooling chambers having a base area covering small portions of the surface of the cells. Together these cooling chambers cover a substantial proportion of the cell surface without any significant space between the cooling chambers. The chambers are adapted to receive a through-flow of a cooling medium which is controlled individually for each cooling chamber. The cooling medium is preferably helium and the extracted heat may be transformed into electrical energy by means of a turbine that drives a generator.

The main idea of that invention is that the cooling medium circulates in one closed loop per individual production cell, thereby the same medium is used both for transport of thermal energy as well as working medium in the engine needed for converting the thermal energy into electrical energy. The closed loop allows for the use of helium under elevated pressure as working medium. This will increase the specific weight and allow for reduced velocity and reduced gas friction for a given rate of cooling. Helium has high thermal capacity pr. mass unit as well as low viscosity and high thermal conductivity compared to air. The rate of heat transfer for a given area and difference in temperature can therefore be high and the friction loss due to transport of the necessary amount of cooling medium through narrow channels will be low. Consequently all cooling areas as well as active areas in heat exchangers can be reduced to minimum size, leaving a maximum portion of the available area for the main purpose, which is production of aluminium.

The most important challenge concerning WO 87/00211 is that the closed circuit including all involved components like heat exchangers, recuperator, compressor, turbine, instruments etc. has to be gas-tight to prevent leakage of working gas to the atmosphere, thus introducing significant costs. Some types of working gas are expensive. If helium is used and the circuit is not absolute leak-proof, accumulated leakage through hardly visible pinholes may over time represent expenses that exceed earnings due to reproduction of electricity as well the benefits from improvements in control of process due to the effective cooling of the cell.

WO 01/94667 discloses an electrolytic cell for the production of aluminium and a method for maintaining a crust on a sidewall and for recovering electricity. At least a part of the sidewall of the electrolytic cell consists of one or more evaporation cooled panels that forms element of a first circulation circuit. Three individual closed circuits plus a unit for dumping low-temperature energy to the atmosphere are specified for a complete solution.

For each of the electrolysis cells is needed:

a) A plurality of primary heat collectors (14 units in parallel are shown in the figure), using liquid metal at boiling temperature as medium for collecting heat energy from the cell.

b) A closed loop circuit, using a preferably inert gas, is used for collecting thermal energy from condensation of evaporated liquid metal in the different units in the first circuit. For the second closed loop, a separate pump is needed for circulating the gas. According to the expressed intention of WO 01/94667 to “convert thermal energy into electricity with an efficiency of 45% or more”, it is obvious that the operating temperature of this pump will have to be high, probably about 800-900° C. and the heat exchangers and recuperator have to be extremely efficient and the inert gas has to be likely helium or hydrogen.

Reliable pumps capable of operating under these conditions represent many technical challenges and may subsequently represent a substantial cost increase both for the installation as well as for the operation.

c) A third closed circuit, (the second closed loop using inert gas) is used for collecting thermal energy from several cells. For the third closed loop, a compressor and an expander combine the pumping function with conversion of the thermal energy into mechanical energy in a rotating shaft also driving an electrical generator. In this third closed loop is also included a recuperator and a cooler for dumping low-temperature energy to the atmosphere.

In accordance with the present invention as defined in the accompanying claims, one or more of the shortcomings, complexity and disadvantages of the prior art can be overcome.

The present invention relates to energy recovery from electrolytic production cells. In particular the invention is suitable for cooling of and heat recovery in electrolysis cells and processes for production of aluminium.

The main objective behind the present invention is the similar as for WO 87/00211 and for WO 01/94667 and is based upon the need for controlled removal of excess heat from electrolysis cells as well as the transformation of a substantial part of this thermal energy into mechanical energy.

The mechanical energy may relatively easy be converted into electricity, compressed air and/or other forms of potential energy that may be used in industrial processes. Similar to that of WO 87/00211, only one and the same working medium is used for both cooling the industrial process and as the working medium in the engine used for conversion of the collected thermal energy into mechanical energy.

However, this is where the present invention differs from prior art: The present invention is based on a self-propelled cooling system using air as the working medium, instead of like in prior art where another working medium is used (preferably an inert gas).

Air is the only cooling medium that can be operated reasonable in an open loop system. For other cooling media different from air, the system will for obvious reasons have to be operated in a closed loop system to prevent major loss of working medium to the atmosphere.

Loss of working medium to the atmosphere will also be a problem if a leak should develop suddenly or over time during operation of a closed loop system. Since the cooling system should operate continually and undisturbed over the operational lifetime of the electrolysis cell, any leak may easily accumulate to loss of larger amount of (expensive) cooling/working media (like Helium and others) and thereby increase the cost of system operation.

While some leaks may be repaired without too many problems, most cracks and leaks related to the cooling chambers and their interconnections that usually will have to be integrated in the side walls of the production cells, are virtually impossible to repair as long as the cell is in operation.

An open loop system is by far less vulnerable to leaks as a closed loop system since the cooling medium in an open loop system always should end up in the surrounding atmosphere. Most cracks and leaks that may occur during the operational lifetime, will have negligible influence on the cooling function of the cells, but may of course reduce the output of recovered energy (electricity, compressed air, etc.).

Air has up to now been avoided as cooling medium inside narrow channels as long as the channels are made of metal and integrated in cooling panels inside side linings for electrolysis cells. The temperature on the inner surface of the cooling channels may occasionally exceed the point where the metallic material will react with the oxygen in the air.

The applicants own, non-published patent application NO 2003 1220 that relates to heat-exchanger panels is based upon the use of ceramic materials such as silicon carbide as contact material in the relatively narrow channels. Since reaction between silicon carbide and oxygen only can take place at higher temperature than the normal process temperature in the electrolysis cells, the use of air should be without problems in this respect. Thus, this type of heat-exchangers may be very suitable for use in accordance with the present invention.

If the diameter and the length of the channels are optimized for the use of air instead of an inert gas, air will collect the necessary amount of thermal waste energy—like if an inert gas is used. Compared with for instance Helium, the use of air will result in only marginal increase in channel diameter, channel length and friction loss.

The most important advantage that is obtained if air is used in an open circuit, is however, the fact that the component preferably used for circulating the air through the cooling channels already exist in the form of turbochargers, typically used for recovering waste energy from the exhaust from diesel engines. Turbochargers have good reputation for reliability and they have long operational lifetime. As they have been available as industrialised components already for a long time, the cost per unit is comparatively low.

The compressor part of the turbocharger compresses the air from ambient temperature and pressure to higher density than the atmosphere, meaning that the velocity of the air through the cooling channels is reduced as well as the friction loss in the channels will be reduced. After the air has passed the cooling channels, the temperature has increased. The friction will result in a loss in pressure and an additional marginal temperature increase before the air enters the expansion turbine of the turbocharger. Under circumstances as described here to cool electrolysis cells, the turbine will produce more mechanical energy than what is needed for operating the compressor.

The recovered excess heat can therefore be converted into pressure energy and in one embodiment into electrical energy. The conversion may take place in a heat-engine, turbine or the similar. The turbine can be applied to drive a compressor, generator or the like.

Alternatively, the heat energy can be applied to produce steam to drive a steam turbine. In one embodiment of the present invention, the recovered energy is fed back to the same cell as it was gathered from. In one other embodiment the energy conversion system serves plural cells or public grid. The heat exchange circuit may preferably be open.

The invention shall be further explained by Figures and examples where:

FIG. 1 discloses a first embodiment where energy is recovered from one aluminium electrolysis cell in an energy conversion unit,

FIG. 2 discloses a second embodiment for energy recovery from one cell,

FIG. 3 discloses a third embodiment for energy recovery including one combustor,

FIG. 4 discloses a forth embodiment for energy recovery comprising two energy conversion units,

FIG. 5 discloses a fifth embodiment for energy recovery from more than one cell, using one (1) energy conversion unit,

FIG. 6 discloses a sixth embodiment for energy recovery from plural cells, using common electrical energy generator

FIG. 7 discloses a seventh embodiment for energy recovery from plural cells, where at least one part of one cold side of the circuit is in common.

As seen in FIG. 1 an aluminium electrolysis cell 1 is provided with conduits 6, 8 for a circulating medium. Conduit 6 is arranged at the cold side of the circuit, while conduit 8 is arranged at the hot side. The medium can be a gas, preferable air but also other gases with acceptable properties may be applied. In the cell, there are arranged heat exchangers (not shown). The heat exchangers can preferably be of the type as comprised in the applicants own patent application NO 2003 1220.

Heated medium from the cell is transported to an expander turbine 3 connected mechanically with a compressor 2 for instance by an axle 4. The outlet of the compressor 2 is connected to conduit 6 preferably via one check valve 7 to circulate medium to the heat exchangers arranged in the cell 1. The compressor may have an inlet 5 that allows ambient air to enter, preferably after being conditioned through a filter and a demister (not shown). At the outlet side of the turbine, there may be arranged a restriction valve 10, before the gas enters one exhaust line 9.

FIG. 2 is based upon the same principles as stated in FIG. 1, but in addition there is arranged one branch with a control valve 20 between the conduits that leads medium to/from the cell 1. The purpose of the branch including valve 20 is to by-pass medium from the cold side directly to the warm side, without traversing the exchangers in the cell and the valve make it possible to control cooling medium flow through the cell 1 and in this way control the cooling effect.

FIG. 3 is based upon the principles as described in FIG. 2. In addition the conduit at the hot side of the cell 1 includes a combustor 30. The combustor can be fed with a gas containing oxygen, such as air via the above mentioned branch. The purpose of the combustor is to elevate the energy level (temperature) of the gas recovered from the cell, to ensure a more stable and efficient operation of the expander turbine(s). The combustor can keep the inlet temperature to the expander turbine on constant level independent of cell outlet temperature. In addition the combustor can be used in cases where electricity price from the grid is high or in case of shortage of electrical energy.

FIG. 4 discloses a forth embodiment based upon the principles as described in FIG. 3. In FIG. 4, the hot side of the circuit leaving the cell 1 have in addition to combustor 3 and turbine 3 with its exhaust line 9, an additional turbine 40 connected with a generator 41 arranged downstream said exhaust line. This embodiment makes possible that recovered electrical energy can be returned back to the cell(s) to support the electrolytic process.

In FIG. 5, there is shown an embodiment based upon the principles of the embodiment as shown in FIG. 3. In addition to the elements of that embodiment, one or more additional cells 51 is connected in parallel to the basic circuit by conduits 52 (cold side) and 53 (hot side) communicating with conduits 6 and 8 respectively. At the cold side connection between said cells 1, 51 there is arranged a three-way valve 50, to control the amounts of circulating medium in the adjacent conduits. The advantage of connecting plural cells to the same energy recovery unit, is that plural cells will contribute to even the actual behaviour of individual cells, and the recovery unit will have more stable operating conditions. Further, there will be possible to reduce the investment costs by applying one larger compressor/turbine unit to serve plural cells. One disadvantage is that it will require more complicated piping. Arrangement of control valves on cold side instead on hot side of the cell will reduce the investments considerably.

FIG. 6 is based upon the principles of the embodiment disclosed in FIG. 5 combining plural cells 1, 51 by conduits 52, 53, and where the hot side conduit 8 comprises combustor, expander turbine with exhaust line 9. The exhaust line is further connected with a second expander turbine 40 that runs one generator 41. This embodiment makes possible that recovered electrical energy can be returned back to the cell(s) to support the electrolytic process.

FIG. 7 discloses an arrangement with two cells 1, 51 interconnected at their cold sides. Both cells 1, 51 have one compressor 2,72 and expander turbine 3, 73 arrangement where the expander turbine drives the compressor via an axle 4, 74. Further the cells have cold side conduits 6, 76 connecting compressor 2, 72 with cell 1, 51 and hot side conduits 8, 78 connecting the hot side of the cells 1, 51 with the turbines 3, 73.

At the cold side of the circuits, in conduits 6, 76, there are arranged three-way valves 81, 82 allowing cold, pressurized surplus medium to be branched off. The branched off cold medium is collected in one conduit 80 and further led to a pressure conversion unit (70) connected with an electrical generator (71). The excessive compressed air of more than two cells may be connected to the conduit 80. The pressure conversion unit may be an expansion engine such as a gas motor, an air turbine or the similar. The advantage of this embodiment is that colder air is transported in the conduit 80 instead of as shown in the previous examples where the bleed-off air has higher temperature. Colder air has less volume per mass unit than hot air. This will reduce the airspeed in the conduct for a given flowpipe-geometry and a given mass-flow. This will reduce the friction losses compared with transport of hot air. Since the temperature is lower, less and cheaper insulation will be needed to keep the thermal losses low.

The heat-engine (turbocharger or compressor/expansion turbine) as disclosed in the above standing embodiments may be dimensioned for compressing the air to an overpressure of typically 3-5 bar. Thereby the temperature will increase from ambient to typically 200-300° C. Since the temperature is higher than ambient, material stress due to temperature shock when the cooling medium enter the cooling channels will be reduced. The compressed air is distributed between a plurality of heat exchangers containing cooling channels, preferably of the type as comprised in the applicant's own patent application NO 2003 1220. The compressed air will pick up excess heat from the electrolytic cell and thereby provide cooling of the side-wall.

Only part of the collected energy will normally be needed to overcome the pressure loss in the cooling channels. There may be also some cracks and minor leakage in some of the channels that will result in pressure loss. When the air is heated, the volume will increase. Therefore more mechanical energy may be produced in the expansion part of the heat-engine than what is needed for running the compressor. This means that only part of the overpressure that exist before the expansion part is necessary to drive the compressor.

We have here the following options:

-   -   a) A generator may be connected to the heat-engine. The expander         will now produce electricity.     -   b) A compressor may be connected, producing compressed air.     -   c) A hydraulic pump may produce hydraulic energy.     -   d) A second expander may be connected in series with the first         expander and thereby divide the total pressure potentials         between the 2 separate units.

Further, it should be understood that the turbocharger unit may preferably be of a commercially available type, similar to those used in heavy duty trucks or ship motors with turbocharged combustion engines. Therefore, by using commercial modules it will be possible to keep the costs at a viable level.

An electric motor-generator (not shown) may be in direct drive or via a transmission with the axle of the turbocharger, to assist the pumping/compressing activity when needed. Such need may occur for instance in start-up conditions, or when large amounts of heat have to be removed from the cell. Further, the generator can be utilised to extract excessive energy when possible. Accordingly, the motor-generator can be controlled by a computer or the similar and thus be used to control the flow of medium through the cell. 

1-14. (canceled)
 15. A method of energy recovery and/or cooling in at least one electrolysis cell (1, 51) for the production of aluminium, where the cell(s) is provided with one or more heat exchangers and where a heat exchange medium circulates through said heat exchanger(s) and is further directed to at least one heat conversion unit, such as an expander turbine (3), wherein the expander turbine (3) is mechanically connected with a compressor (2) that supplies an oxygen containing medium, in particular ambient air, at elevated pressure to the cell (1, 51) for heat exchange and cooling of the cell in a substantially self propelling manner, and where the heat is extracted by heat exchanger(s) consisting of a material that is substantially inert with respect to oxygen at its operating pressure and temperatures.
 16. A method in accordance with claim 15, wherein the heat exchange medium is air taken from the surroundings.
 17. A method in accordance with claim 15, wherein the heat exchange medium after passing through the heat conversion unit is let out to the surroundings.
 18. A method in accordance with claim 15, wherein the heat exchange medium is led through a combustor (30) for increasing the temperature thereof before entering at least one heat conversion unit (3, 40).
 19. A method in accordance with claim 15, wherein the surplus of cold heat exchange medium is led through valves (81,82) before entering heat/pressure to electricity conversion unit (70).
 20. A system for energy recovery and/or cooling in at least one electrolysis cell (1,51) for the production of aluminium, where the cell has one or more heat exchangers and where a heat exchange medium circulates through said heat exchanger(-s) and is further directed to at least one first heat conversion unit, such as an expander turbine (3), wherein the expander turbine (3) is mechanically connected via one axle with a compressor (2) that supplies an oxygen containing medium, in particular ambient air, at elevated pressure for heat exchange to the cells (1, 51), and where the heat exchanger(-s) is made out of a material that is substantially inert with respect to oxygen at its operating pressure and temperatures.
 21. A system in accordance with claim 20, wherein the heat exchange medium is air that circulates in one circuit (5, 6, 8, 9) that is open with respect to the surroundings.
 22. A system in accordance with claim 20, wherein a combustor (30) is arranged between the cell(-s) (1, 51) and the heat conversion unit (3).
 23. A system in accordance with claim 20, wherein one second expansion turbine (40) is arranged downstream the first heat conversion unit (3).
 24. A system in accordance with claim 23, wherein the second turbine (40) is connected with one generator (41), for electricity production.
 25. A system in accordance with claim 24, wherein the electricity produced is reverted back to the cell(s) or the public grid.
 26. A system in accordance with claim 20, wherein cold medium pressurized by the compressor(-s) (2, 72 or others) is expanded in an expansion engine (70) that preferably drives one generator (71).
 27. A system in accordance with claim 20, wherein the material of the heat exchanger(-s) comprises a ceramic material
 28. A system in accordance with claim 20, wherein the material of the heat exchanger(-s) comprises silicon carbide.
 29. A system in accordance with claim 20, wherein a motor-generator is mechanically connected with the expander turbine (3) and the compressor (2).
 30. A system in accordance with claim 20, wherein the pressure of the oxygen containing medium that enters the heat exchangers is between 3 to 5 bars.
 31. A system in accordance with claim 20, wherein the temperature of the oxygen containing medium that enters the heat exchangers is between 200 and 300° C. 